* {
    margin: 0;
    padding: 0;
}

.box {
    display: flex;
}

.left {
    height: 100vh;
    width: 400px;
    background-color: aqua;
    z-index: 10;
}

.right {
    height: 100vh;
    flex: 1;
    background-color: burlywood;
    z-index: 9;
}